Block reads and writes

A block read is a set of attributes that can be used to retrieve values in a single transaction. Instead of the I/O points being on advise, as with a scan group, the attributes are updated once per transaction. A block read must be initiated from a user or script via the BlockRead.TransactionTrigger attribute.

A block write is a set of attributes that can be used to write values in a single transaction. Only values that are new for the attribute since the last transaction are written to the OPC communications driver. A block write must be initiated from a user or script via the BlockWrite.TransactionTrigger attribute. You cannot trigger another transaction if the previous transaction has not completed.

Note: Although block reads and writes are intended to be individual transactions, the actual communication with the field device is dependent on the number and type of attributes, and it may be necessary to send and receive multiple messages to and from the field device. As a result, the values exposed after a block read operation might have been obtained at different times. You must carefully design the logic in the field device to make sure that the values do not change until the full read operation is completed. In the case of a block write operation, the logic in the field device must not use the written values until the full write operation is completed.

For a block read, either all of the attributes will be updated or none of the attributes will be updated; no partial updates are supported. In the case of a block read failure or transaction timeout, the data quality for all attributes is set to BAD.

For a block write, partial writes are supported. However, the system does not offer roll-back functionality. In the case of a block write failure or transaction timeout, some values may have been successfully sent, but there is no way to undo the value changes. For items that have not yet been written to, the data quality is set to BAD.

Finally, runtime object attributes allow you to monitor errors related to the data quality for item values in a block read or block write operation.